<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box {

            margin: 50px auto;
            display: flex;
        }

        .box div {
            border: 1px solid black;
            height: 500px;
        }

        .left {
            flex: 1;
        }

        .center {
            flex: 300px;
        }

        .right {
            /*是左边的两倍宽*/
            flex: 2;
        }
    </style>
</head>
<body>
<div class="box">
    <div class="left">自适应</div>
    <div class="center">固定宽度</div>
    <div class="right">自适应</div>
</div>
</body>
</html>